A radio frequency identification (RFID) communication system is a wireless system having a reader and at least one tag. Communication interfaces between the reader and the tag have been described in, for example, ISO/IEC 18000-3, mode 2.
Each tag typically has a single chip with a memory, which stores data, and a radio transceiver or transponder. One tag format is a chip mounted on a plastic sheet that contains an antenna in the form of a small loop.
The reader, also called an interrogator, is a higher-power transceiver having a larger wideband antenna to interrogate the tag with an interrogation or carrier signal. Working with a computer system, such as a point of sale system or inventory control system, the reader stores the tag information and takes appropriate action, depending upon the application. It is also possible for the reader to write to the tag and change the tag's data.
A tag can be passive, semi-passive, or active. In the passive case, the tag obtains power from the RF signal received from the reader to power its circuitry. That is, a passive tag rectifies and filters the received signal into a DC signal that powers the circuitry. An active tag differs from a passive tag in that it has its own power source for boosting transmit power and storing more information.
RFID systems use transformer theory, with the reader antenna being the primary winding of the transformer, and the tag antenna the secondary winding. Signal strength at the tag is proportional to its distance from the reader, and thus the tag has to be placed in close proximity to the reader in order for the tag and the reader to communicate. A typical range is several inches, but the range varies with the frequency of operation as well as antenna size.
Data transfer methods in RFID systems vary depending on the application and type of coupling. Currently most RFID systems use amplitude shift keying (ASK). Another data transfer method is phase shift keying (PSK). When the reader is turned on, the tag powers up and waits for reader commands to be processed. Binary pulses representing the reply data modify the impedance of the tag's antenna, which in turn causes an amplitude shift in the carrier signal. The process loads and unloads the secondary winding of the tag to reflect an impedance back into the primary winding of the reader via the modulated carrier signal. The result is an AM signal decoded into a serial data signal by the reader.
RFID systems feature anti-collision resolution, as multiple tags powered up simultaneously within the reader's antenna field can interfere with one another. Many available schemes prevent such collisions. One scheme uses a time-division multiplexed arrangement, assigning each tag a time slot in which to transmit. Also, collisions can be resolved by muting all tags except the tag being read to ensure that no collision occurs. After a certain period, the muted tags are reactivated.
The response signal strength level of the tag is typically a low level to allow multi-channel operations, e.g. eight channel operations, with minimal crosstalk between channels. This low response signal strength level requires expensive and highly sensitive readers to detect the tag's transmissions. Furthermore, the above described low response signal strength level is also used in single channel mode where crosstalk is nonexistent, thereby reducing the communication range of the system and resulting in potential loss of data when a tag is at or near the threshold of the communication range.